METHODS OF MEASURING ARTERIAL BLOOD PRESSURE 



211 



have some arrangement by means of which the mercury may be made to 

 rise in the tube of the manometer to the level corresponding to approxi- 

 mately the mean pressure of the artery experimented with, so that the writing 

 style simply records the variations of the blood pressure above and below 



FIG. 185. Arterial Cannula. T-form for convenience in washing out clots. 



the mean pressure. This is done by causing the saline solution, generally 

 a saturated solution of sodium carbonate or of 10 per cent, magnesium sul- 

 phate, to fill the apparatus from a bottle suspended at a height about that 

 of the pressure to be measured, and capable of being raised or lowered as 



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*< 1 tvv * t * 1 1 1 1 1 1 * t 1 * t ***-* * +*-** t t ft **-*-* 



FIG. 186. Tracing of Normal Arterial Pressure in the Dog, Obtained with the 

 Mercurial Manometer. The smaller undulations correspond with the heart-beats; the 

 larger curves with the respiratory movements. Pressure is in millimeters of mercury as 

 shown by the scale to the left. Time in seconds. (New figure by March and Nugent.) 



required for the purpose. The cannula inserted and tied into the artery 

 may be of several different kinds. A glass T-tube with the end drawn out 

 and cut so that it is oblique, and provided with a slightly constricted neck 

 to prevent its coming out of the artery easily, is a very convenient form, 



